Here’s the other video I alluded to in the vCenter upgrade post. Here’s the post with the video for upgrading ESXi 3.x to vSphere 4.0. Remember, you’ll need to upgrade your vCenter server first and then you can upgrade your ESX servers using Update Manager. There is also the vSphere Host Update Utility method, which is installed when you install the vSphere Client. This is also a simple way of doing your host upgrades. I’ll try and find and post videos or something using that method.

http://vmwaretips.com/wp/2009/06/07/upgrade-esxi-3x-to-40-video/